Reflectance data has a variety of uses. The thickness of the various coatings (either single layer or multiple layer) on the wafer can be determined from a reflectance or relative reflectance spectrum. Also, the reflectance at a single wavelength can be extracted. This is useful where the reflectance of photoresist coated wafers at the wavelength of lithographic exposure tools must be found to determine proper exposure levels for the wafers, or to optimize the thickness of the resist to minimize reflectance of the entire coating stack. The refractive index of the coating can also be determined by analysis of an accurately measured reflectance spectrum.
It is especially useful, for a variety of industrial applications, to measure the thickness of a very thin film (less than about 300 angstroms in thickness) on a sample, by reflectance measurements of the sample under a microscope. For example, the sample can be a semiconductor wafer, and the very thin film can be coated on a silicon substrate of the wafer.
Because of the tight tolerance requirements typically required in the semiconductor arts, an accurate means for obtaining reflectance measurements of a wafer is needed. In conventional reflectance measurement systems, monochromatic or broadband radiation is reflected from the wafer, and the reflected radiation is collected and measured.
FIG. 1 is a broadband small spot spectral reflectometer, camera, and autofocus apparatus, over which the present invention represents an improvement. The FIG. 1 system includes an optical system for measuring reflectance of a sample 3 (which is typically a wafer), and means for focusing sample 3 with respect to the optical system, including an illumination subsystem, a reflectometer subsystem, a viewing subsystem, and an autofocus subsystem, wherein any given optical element may be part of more than one subsystem.
The illumination subsystem includes lamp 10 (typically a xenon arc lamp) which emits radiation beam 12 (comprising visible and/or UV radiation), lamp housing window 14, off-axis paraboloid mirror 16, flip-in UV cutoff filter 18, color filter wheel 20, a flat mirror 22, a concave mirror 24, aperture mirror 28 with flip-in forty-micron fine focus aperture 30, large achromat 32, field illumination shutter 31, fold mirror 36, and small achromat 38.
The illumination system provides combined beam 42 comprising measurement beam 25 and field illumination beam 34. Lamp 10 emits beam 12 through lamp housing window 14. Window 14 is provided to contain lamp 10 should the lamp crack and explode. Although window 14 is not necessary for optical reasons, it can function (with lamp heatsink window 14a, and with battles not shown in FIG. 1) to keep lamp cooling air from being drawn through the optical path (since such flowing air could otherwise cause shimmering of the arc image and contribute to noise). A xenon lamp is preferred over other lamps such as tungsten or deuterium lamps, because a xenon lamp will give a flatter output covering a spectrum from UV to near infrared. Alternatively, a tungsten lamp and a deuterium lamp can be used in combination to cover the same spectrum covered by a xenon lamp, but this combination still leaves a gap in brightness in the mid-UV wavelengths. Brightness of the spectrum is important, because with less intensity, reflected radiation must be collected for longer periods, thus lower intensities slow the measurement process.
Off-axis paraboloid mirror 16 collimates beam 12, which can be optionally filtered by flip-in UV cutoff filter 18 and color filter wheel 20. Flip-in UV cutoff filter 18 is used in part to limit the spectrum of beam 12, so that when beam 12 is dispersed by a diffraction grating, the first and second order diffraction beams do not overlap. Part of beam 12 is reflected by flat mirror 22 onto concave mirror 24 to form measurement beam 25. Optionally, concave mirror 24 can focus beam 25 onto the end of a large-core silica fiber which acts as a radial uniformer (in an effort to attain a radially symmetric beam cross-section at the output of the large-core silica fiber regardless of the intensity pattern in a cross section of measurement beam 25 at the input of the large-core silica fiber). Without such a radial uniformer, it is possible that the arc in lamp 10 might shift and cause the intensity of light across a cross section of measurement beam 25 to shift causing apparent fluctuations in the relative reflectance spectrum determined from the output of photodiode arrays 72 and 74. However, in many applications it is preferable to omit the large-core fiber. Without the large-core fiber, mirror 24 focuses an image of the arc onto aperture mirror 28. The radiation emanating from each point on the image of the arc expands in a uniform cone, typically producing a uniform circle of illumination at beam divider 45.
Another part of beam 12, field illumination beam 34, is focused by large achromat 32 near fold mirror 36, causing fold mirror 36 to reflect an image of lamp 10 toward small achromat 38. Small achromat 38 collects the radiation in beam 34 before it reflects from aperture mirror 28. Aperture mirror 28 is preferably a fused silica plate with a reflective coating on one side, with a 150 micron square etched from the reflective coating to provide an aperture for beam 25. The aperture is placed at one conjugate of objective 40. The field illumination can be turned off by placing field illumination shutter 31 in the optical path of field illumination beam 34.
Narrow measurement beam 25 and wide field illumination beam 34 are rejoined at aperture mirror 28, with field illumination beam 34 reflecting off the front of aperture mirror 28, and measurement beam 25 passing through the aperture. The use of flip-in fine focus aperture 30 is explained below. The reflectometer, viewing, and autofocus subsystems of the FIG. 1 system include objective 40, beamsplitter mirror 45, sample beam 46, reference beam 48, concave mirror 50, flat mirror 43, reference plate 52 with a reference spectrometer pinhole therethrough, sample plate 54 with a sample spectrometer pinhole therethrough, second fold mirror 68, diffraction grating 70, sample linear photodiode array 72, reference linear photodiode array 74, reference photodiode 95, sample photodiode 93, an achromat with a short focal length and a right angle prism (not shown), beamsplitter cube 84, penta prism 86, achromats 88 and 90 with long focal lengths, third fold mirror 89, focus detector 98, neutral density filter wheel 97, fourth fold mirror 91, and video camera 96.
Objective 40, which can be a reflective objective (as shown in FIG. 1) or a transmissive objective (as shown in FIGS. 3-6), preferably has several selectable magnifications. In one embodiment, objective 40 includes a 15.times. Schwarzchild design all-reflective objective, a 4.times. Nikon CFN Plan Apochromat (color corrected at three wavelengths), and a 1.times. UV transmissive objective, all mounted on a rotatable turret which allows for one of the three objectives to be placed in the optical path of sample beam 46.
The measurement of the relative reflectance spectrum of wafer 3 will now be described. When field illumination shutter 31 is placed in the path of field illumination beam 34, combined beam 42 comprises only measurement beam 25. Combined beam 42 is split by beamsplitter mirror 45, a totally reflecting mirror placed so as to deflect half of combined beam 42 towards objective 40, thus forming sample beam 46, the undeflected half of combined beam 42 forming reference beam 48. Importantly, because sample beam 46 and reference beam 48 are derived from the same source (lamp 10) and because combined beam 42 is radially uniform, reference beam 48 and sample beam 46 have proportionally dependent spectral intensities. Furthermore, since beamsplitter mirror 45 is a totally reflecting mirror in half of an optical path rather than a partially reflecting mirror in the entire optical path, a continuous broadband spectrum is reflected with good brightness.
Reference beam 48 does not initially interact with beamsplitter mirror 45, but instead illuminates concave mirror 50. Concave mirror 50 is slightly off-axis, so reference beam 48 is reflected onto the reverse face of beamsplitter mirror 45, and flat mirror 43 re-reflects reference beam 48 into alignment with the reference spectrometer pinhole through plate 52. Flat mirror 43 is provided to realign reference beam 48 with sample beam 46 so that both beams pass through their respective spectrometer pinholes substantially parallel.
The focal length of concave mirror 50 is such that reference beam 48 is in focus at the reference spectrometer pinhole (which extends through plate 52). The radiation passing through the reference spectrometer pinhole and reflecting from fold mirror 68 is dispersed by diffraction grating 70. The resulting first order diffraction beam is collected by reference linear photodiode array 74, thereby measuring a reference relectance spectrum.
Sample beam 46 is reflected from beamsplitter mirror 45 towards objective 40, which focuses sample beam 46 onto wafer 3, and the reflected sample beam 46 is focused by objective 40 onto the sample spectrometer pinhole (which extends through plate 54). The reflected sample beam 46 does not interact with beamsplitter mirror 45 on the reflected path, because sample beam 46 passed through the space behind beamsplitter mirror 45, through which reference beam 48 also passes. The radiation passing through the sample spectrometer pinhole and reflecting from fold mirror 68 is dispersed by diffraction grating 70. As with the reference beam, the resulting first order diffraction beam of the sample beam is collected by sample linear photodiode array 72, thereby measuring the sample spectrum.
The relative reflectance spectrum can be simply obtained by processing the outputs of arrays 72 and 74 in processor 100, by dividing the sample light intensity at each wavelength (the output of array 72) by the reference intensity at each wavelength (the output of array 74). Typically, this involves 512 division computations, in cases in which each of arrays 72 and 74 is a 512-diode linear photodiode array. A typical relative reflectance spectrum will include components ranging from 220 nm to 830 nm.
In some embodiments, diffraction grating 70 is a concave holographic grating and the spectrometer pinholes (through plates 52 and 54) are 15 mm apart. This embodiment of diffraction grating 70 is holographically corrected to image multiple spectra, since the 15 mm spacing does not allow for both beams to be centered on the grating. One such grating is a multiple spectra imaging grating supplied by Instruments S.A. It is also desirable that grating 70 be designed so that the angle of detectors 72 and 74 causes reflections from the detectors to propagate away from the grating.
The FIG. 1 system includes an autofocus subsystem having a coarse-focus mode to allow for wide range lock-in, and a fine-focus mode for use once a coarse focus is achieved. In the coarse-focus mode, flip-in fine-focus aperture 30 is flipped out of the optical path, and the square aperture of aperture mirror 28 is imaged onto detector 98. Variations on the FIG. 1 system may not implement the coarse-focus mode.
Detector 98 has a position output, which is dependent on the position of the centroid of the radiation falling on detector 98, and an intensity output, which is dependent on the incident intensity at detector 98. Detector 98 is also positioned to avoid dark regions of the out-of-focus image. In the coarse-focus mode, the centroid of the image falling on detector 98 indicates not only the direction in which focus lies, but also how far out of focus wafer 3 is. The Z position of wafer 3 (the separation between water 3 and objective 40) is then adjusted until the centroid of the light falling on detector 98 is centered near the center of detector 98. With the appropriate positioning and feedback mechanism, wafer 3 can be kept in coarse focus while the wafer is being moved in the X and Y directions. In one embodiment, a feedback loop between detector 98 and a servo motor which adjusts the focus is disabled when no light falls on detector 98. This is to prevent uncontrolled movement of the stage supporting water 3.
For fine focus, flip-in aperture 30 is flipped into the optical path of measurement beam 25, resulting in a smaller square image reaching detector 98. The smaller square image has a size of about 40 microns with a IX objective. Since flip-in aperture 30 is the same size as the aperture though plate 54, and since the two apertures are at conjugates of objective 40, when wafer 3 is in focus, very little radiation strikes plate 54 (away from the aperture through plate 54) to be reflected onto detector 98. Thus, in the fine-focus mode, the intensity output of detector 98 is used to bring wafer 3 into focus, with the Z position of wafer 3 being adjusted until the intensity output of detector 98 is minimized.
In an operating mode for measuring the thickness of very thin film 3a (VTF 3a) on sample 3, the FIG. 1 system employs sample VTF photodiode 93 and reference VTF photodiode 95. Dichroic mirror 152 mounted on a moveable arm flips into the beam path immediately beyond apertured plates 52 and 54. The dichroic mirror reflects UV radiation (with wavelength between 400 nm and 280 nm) and transmits visible light. The reflected UV from the reference beam is focused by fused silica lens 155, reflected by fixed dichroic mirror 156, and finally falls on UV enhanced silicon photodiode 95 (the "reference VTF photodiode"), and the reflected UV from the sample beam is focused by fused silica lens 153, reflected by fixed dichroic mirror 156, and finally falls on UV enhanced silicon photodiode 93 (the "sample VTF photodiode"). Second dichroic mirror 156 is needed to filter out residual visible light. The radiation transmitted through first dichroic 152 continues through the normal spectrometer path.
Each of photodiodes 93 and 95 measures a single intensity value, but typically this value is an average over a broadband frequency range of interest (in the UV range) so that the two photodiodes provide sufficient information for calculating a relative reflectance (or reflectance) value representing an average over such broadband frequency range. Photodiodes 93 and 95 are preferably selected to have sensitivity to a broad range of wavelengths in the UV band, with both photodiodes having substantially similar peak sensitivity wavelengths. When the response of sample photodiode 93 is divided by the response of reference photodiode 95, the result is a value indicative of the relative reflectance of wafer 3 over wavelengths in the UV band, with the peak sensitivity wavelength having more weight in the measure of relative reflectance than other wavelengths. The measured relative reflectance value can be calibrated to generate a signal indicative of the true reflectance of the sample in the UV band.
The reasons for employing photodiodes 93 and 95 to measure the thickness of a very thin film (VTF) on wafer 3 are as follows.
When a relatively thick film (having optical thickness greater than 1/4 the wavelength of the illuminating radiation) is deposited on a reflective substrate, the interference between light reflecting from the top and bottom interfaces creates maxima and minima in the reflectance spectrum. The thickness can be determined by finding the wavelength position of the extrema, or by finding the best match between the shape of the measured spectrum and the shape of theoretically calculated spectra of different thicknesses.
As the film becomes thinner, the number of extrema are reduced. With films so thin that the last minimum has disappeared, there is a thickness range of a couple of hundred angstroms in which the curvature of the spectrum still provides important information about the film thickness, but the absolute intensity of the spectrum becomes increasingly more important as the film becomes thinner. Very thin films (films having optical thickness much less than 1/4 the illuminating wavelength) have reflectance spectra that are just a few percent dimmer than the reflectance spectrum of a bare substrate. Thus, when measuring thick films, errors in the wavelength scale of the spectrum are more significant, whereas with very thin films the vertical scale (absolute intensity of measured reflectance) should be measured accurately.
The radiation incident on photodiodes 93 and 95 has propagated through an optical path that bypasses grating 70 to avoid the "grating tilt effect," and each of photodiodes 93 and 95 preferably receives a broad range of UV wavelengths. The grating tilt effect is that a change in orientation (tilt) of wafer 3 will cause radiation falling on grating 70 to shift using a slightly different portion of the surface of grating 70. Typical concave gratings used to implement grating 70 have sharply varying efficiencies across their surfaces, and so a change in sample tilt causes an undesirable change in the signal diffracted by grating 70.
The UV radiation incident on photodiodes 93 and 95 is preferably not filtered through a narrow band filter (because if it were, too little radiation would reach the photodiodes). To avoid the need to apply a complicated algorithm (assuming a weighted average of many incident wavelengths) to compute film thickness, processor 100 of the FIG. 1 system determines a single effective wavelength for the broadband UV incident on each of photodiodes 93 and 95. The analog output of photodiodes 93 and 95 is digitized (and otherwise processed) in electronic circuitry 90 before undergoing digital processing in processor 100.
Although measurements made using photodiode 93 alone may be sufficient to measure film thickness in some contexts, additional measurements are usually made using photodiode 95 in the reference beam path, to correct for lamp noise.
The processing steps performed on the output of photodiodes 93 and 95 are described in detail below. These processing steps are briefly summarized in this paragraph. Before each sample 3 is measured, darknoise is measured for each of photodiodes 93 and 95. Darknoise for the sample channel is the DC offset occuring with no sample present on the sample stage (representing stray light and cross talk from the reference path, which is to be subtracted from sample measurements), and darknoise for the reference channel is the DC offset occuring with an opaque disk in the optical path at the location of color filter 20 (e.g., an opaque disk of a color filter wheel implementation of color filter 20). Darknoise is subtracted from the output signal of each of photodiodes 93 and 95, and the sample channel signal (the corrected output of photodiode 93) is then divided by the reference channel signal (the corrected output of photodiode 95) to yield a reading that is proportional to sample reflectance but largely independent of lamp intensity fluctuations.
As noted, photodiodes 93 and 95 are preferably sensitive to a range of UV wavelengths, with photodiode 95 receiving reference UV radiation and photodiode 93 receiving UV radiation that has reflected from the sample. There are two advantages to measuring very thin films with UV radiation rather than visible radiation. The first is that, since the wavelength range of UV radiation is shorter than that of visible light, the optical thickness of the measured film expressed in units of wavelengths is actually greater. The second advantage applies mainly to measurements of films on silicon substrates. There is a sharp change in the complex refractive index of silicon that occurs near 400 nm. If reflectance versus thickness is plotted for silicon dioxide on a silicon substrate (the most common VTF) for a single visible wavelength, the curve is fiat (zero derivative) at zero thickness, but drops and becomes increasingly steeper for greater thicknesses. The same curve (for UV radiation) has a non-zero derivative (slope) at zero thickness. Thus, sensitivity to changes in the thickness of this type of VTF near zero film thickness does not drop nearly as quickly in the UV as it does in the visible.
Although the design of the FIG. 1 system substantially reduces many sources of error, since it includes means for automatically keeping the sample in focus and since its optics are designed to reduce sensitivity to sample tilt and variation in the illuminating radiation's intensity and spectrum, several difficulties arise when operating the FIG. 1 system to perform VTF measurements (using photodiodes 93 and 95).
Although the FIG. 1 system uses UV radiation to measure very thin film thickness, it must measure the absolute reflectance to better than 0.05% to obtain the precision and stability required in many applications.
Although the FIG. 1 system can measure thickness of a 30 Angstrom (30 A) film on a substrate with a precision of less than 0.40 A (one sigma), and a stability less than 1.40 A (one sigma), this degree of precision and stability is inadequate for some applications. One contribution to instability is drift between a wafer sample and reference path during measurement. By measuring a reference sample (through the sample channel; not the reference channel) just before measuring the wafer (or other sample) of interest, it is possible to correct for any drift that had previously occurred, but if the test requires many sites to be measured (or if the room temperature varies quickly) the drift between measurements on the first and last sites on the wafer can be significant.
A second effect contributes even more to measurement error. The surface of a thin coating on a sample typically has nonuniform distance from objective 40, so that such distance varies from point to point on the sample surface. Typically, this is due to variation in the thickness of the sample substrate, with a uniformly thick coating following the substrate contours. Variation ("micro-ripple") in the position of the upper and lower surface of the coating (relative to objective 40) over a small area of the sample can act as a small lens to defocus radiation as the radiation reflects from the sample.
A third source of error is that the effect of two hertz lamp noise sometimes cannot be reduced to an acceptable level. Lamp noise can be exacerbated by many things, such as alignment, air currents, and the like, but the basic problem is that the reference path collects radiation from the illuminator at a different angle than the sample path.
A fourth source of error is that a high power (e.g., 15.times.) objective lens can be so sensitive to focus errors that the described auto-focus subsystem cannot effectively correct such errors.
Examples of other conventional film thickness measurement systems include those described in U.S. Pat. No. 5,241,366 issued Aug. 31, 1993 to Bevis et al., and U.S. Pat. No. 4,645,349, issued Feb. 24, 1987 to Tabata. The Tabata system determines thickness of a film from a measured reflectance spectrum. A broadband radiation source (16) illuminates a monochromator (19), which, through a partially reflecting mirror (22), illuminates a film (31). The monochromator filters the broadband radiation by reflecting it off a diffraction grating (20), and the monochromator output wavelength is selected by rotating the diffraction grating with respect to a directional mirror. A reflected beam from the film is reflected back along the original optical path, and is reflected out of the original optical path by the partially reflecting mirror. The reflected beam then illuminates a photo-multiplier tube (26), and the output of the photo-multiplier tube is connected to a graphics device (30), which is also connected to a wavelength output of the monochromator, allowing the graphics device to display a graph of reflectance versus wavelength. However, since a scanning monochromator is used, obtaining the reflectance spectrum is time consuming, and no means is provided to ensure that the intensity of the incident radiation is uniform over the time period of measurement. Furthermore, the system of Tabata assumes the sample is in focus. If the sample is not in focus, the reflected radiation may not be sufficiently focused by the objective to provide a useful spectrum. The optics also present special problems, because the diffraction grating must be precisely aligned with the directional mirror. The partially reflecting mirror is also difficult to manufacture with good efficiency when a very wide range of wavelengths are to be used. Even in the best case, the losses due to the partially reflecting mirror are squared, as the radiation must pass through the element twice.